The present disclosure relates to a method for detecting and monitoring electrical signals from the heart for analysis of heart malfunctions. More particularly, the presently disclosed method relates to a technique for distinguishing between ischemic electrocardiograph (ECG) events and patient positional changes during patient monitoring.
Multi-lead ECGs for diagnosing various heart maladies have been used for many years. The most prevalent technique for analyzing and diagnosing heart conditions involves a 12-lead system. The 12-Lead system provides much redundant information in the frontal plane (X, Y) and transverse plane (X, Z) of the ECG vector signal. It permits only a rough visual estimate of the vector direction in theses two planes. Other techniques such as two-dimensional (2-D) vector cardiograms have proven in the past to be quite expensive and cumbersome due to the relative cost and size of the equipment needed to properly display the vector cardiograms, e.g., one cathode ray tube (CRT) oscilloscope was needed for the display of each bodily plane. Further, analysis of the 2-D vector cardiograms typically required a high degree of technical skill and mental agility in interrelating the three displays to formulate a good picture of the 3-D vector. Rules were established on the basis of individual 2-D diagrams and the 3-D vector effect was lost. As a result, the 12-Lead system has become prevalent and widely accepted.
However, in order to teach the 12-lead system, it has always been important for doctors to have a rudimentary knowledge of the relation of the ECG signal to the electrical activity of the heart. This relation is briefly summarized below.
The heart pulse is initiated by the Sino-artial (S-A) node which is generally located in the right atrium and, in a normal heart, acts as the heart's chief pacemaker. The stimulation or depolarization of the entire atria takes place after the occurrence of the S-A node pulse. A graphical representation of the initial depolarization of the atria on the electrocardiogram is represented by a positive deflection on the ECG and is commonly called the P-wave (See FIG. 12).
After an initial electrical pulse from the S-A node, depolarization of the heart muscle spreads to the atrioventicular (A-V) node and is then conducted to the “Bundle of His” (during which time it is slowed down to allow for the atria to pump blood into the ventricles) and thereafter to the “Bundle Branches”. This is known as the PR Segment. The P-R Interval represents the time of transmission of the electrical signal from the initial S-A node impulse to the ventricles.
Ventricle depolarization is known on an ECG by the QRS complex which relates to the contraction or depolarization of the heart muscles, in particular, the right ventricle and left ventricle. This is the most studied cycle and is considered to be the most important for the prediction of health and survivability of a patient. It is initiated by the signal from the Bundle of His and then the high speed Purkinje muscle fibers rapidly excite the endocardium of the left ventricle and then the right ventricle. Early experimental work showed the timing of this excitation and the progress of the electrical wave through the right and left ventricles of the heart, however, it was very difficult to determine the net vector effect of this 3-D wave and its relationship to the overall movement of the cardiac muscle. As a result, most textbooks and physicians have adopted a simplified two-dimensional approach to analyzing this problem.
On the graph shown in FIG. 12, ventricular depolarization is clearly discernible. The most easily recognizable deflection (positive deflection—upward movement above the base line on the ECG) of the QRS complex is termed the R-wave. Just prior to this deflection is the Q-wave which is typically represented by an inverted signal deflection (negative deflection—downward movement below the base line on the ECG). The negative deflection after the R-wave is termed the S-wave which is the terminal part of the QRS complex. (See FIG. 12).
Repolarization occurs after the termination of the S-wave and starts with another positive deflection know as the T-wave. The time frame for the initiation of repolarization is termed the S-T segment and is usually represented by an isoelectric signal, i.e., neither positive or negative deflection. This S-T segment is a most important indicator of the health of the ventricular myocardium.
In order to show these electrical signals as they activate and stimulate the heart muscle, a system had to be developed to record the signals as they transverse the cardiac muscle. Einthoven found that by placing electrodes at various positions on the body and completing the circuit between the heart muscle and the electrocardiogram, it was possible to view the electrical activity between two electrodes of the heart. Each view derived from the varying placement of the electrodes was known as a “Lead”. For most purposes, a typical ECG screening involves using a 12-Lead system in which the leads are arranged at various points of the body, e.g., outer extremities, and the signals are recorded across each “Lead”. A physician is trained to analyze and interpret the output from these Leads and make a diagnosis. In order to help a physician make an accurate diagnosis, various formulas and methods have been developed which translate the output of the 12-Lead system into workable solutions, e.g., Einthoven's Law and 2-D Vector Cardiography.
In order to better explain the novel aspects and unique benefits of the present invention, a brief explanation of vector cardiographic analysis and the numerous steps and processes a physician typically undergoes in order to offer a somewhat accurate diagnosis is relevant.
Vector Cardiography uses a vector description of the progress of the signal through the heart during a QRS interval. This vector representation forms the basis upon which a doctor is trained to understand and explain the outputs received at the various electrodes in the 12-Lead system. Typically within a period of about 0.08 seconds (one normal QRS interval), both ventricles are depolarized and, as a result an electrical force is generated which is characterized by a vector which depicts both the size and direction of the electrical force. In electrocardiography, these vectors are created sequentially over the entire QRS interval. The normal plane for these vectors (i.e., the normal plane of activation) is the same as the QRS cycle, i.e., perpendicular to the X, Y plane (frontal) and slanted along the axis of the heart.
In actuality, the muscle depolarizes from cell to cell and forms an electrical wave front (a plane which separates tissue of different electrical potential) as a function of time. This wave front can be used to determine the resultant or mean vector whose magnitude, direction and location can be determined by the summation of all the small vectors which can be drawn perpendicular to the wave front. The resultant or mean vector of all these vectors is the resultant vector which is measured by the external electrodes and is called the QRS vector. As can be appreciated, other mean vectors are created over the other intervals in the ECG cycle in much the same manner are termed appropriately, namely, the mean T-vector and the mean P-vector.
Traditionally, it has been found that the force and direction of the QRS vector would give an accurate representation of how the heart was functioning over the period of the QRS interval. In order to help determine the QRS vector in the frontal plane, a law was developed by Einthoven which interrelated three (3) electrodes specifically oriented on the body (right arm, left leg and left arm). The signals between each two of the electrodes constituted a “Lead”. These leads formed a triangle known as Einthoven's triangle and it was that these Leads could always be related to a single vector in the frontal plane, i.e., any two signals when added vectorally give a third vector. For diagnostic purposes these Leads were later graphically translated into a triaxial system. Other Leads were subsequently added to the triaxial system (i.e., termed unipolar leads—aVR, aVL, and aVF) and a Hexial system was developed. For simplification purposes, the system was displayed out on a circle and degrees were later assigned to the various leads of the system. FIG. 1a shows the circle which was developed to represent the six Leads. This system is highly redundant.
In order for a physician to determine the mean QRS vector, the physician would line up the various leads around the circle according to their positivity or negativity and mark the transition from positive to negative on the circle. This area of transition is typically referred to as the “transition” area which when analyzing a single plane, e.g., the frontal plane, is represented by a line on the circle which separates the circle into positive and negative halves. (See FIG. 1b). The mean QRS vector is positioned at a right angle to the transition line on the positive side. (See FIG. 1b).
Using the above methodology, the direction and location of the mean QRS vector on the circle determines how the heart is functioning and allows a physician to ascertain typical heart malfunctions. For example, in a normal adult, the mean QRS vector is usually located between 0° and 90°, i.e., between leads I and aVF on the circle. However, a left axis deviation (LAD) is characterized by the mean QRS vector being located in the 0° to −90° area and with right axis deviation (RAD) the mean QRS vector is located in the 90° to 180° area.
The mean T-vector and the mean P-vector are determined in a similar manner. In fact, physicians have determined that one of the more important elements of graphically illustrating the means QRS vector and the mean T-vector is that the angle between the two vectors can be easily ascertained. This angle relates the forces of ventricular depolarization with the forces of ventricle repolarization. In a normal adult, the angle between the mean QRS vector and the mean T-vector is rarely greater than 60° and most often below 45°.
Similarly, the mean P-vector can be determined. This enables a physician to isolate the location of the electrical direction of the excitation of the cardiac muscle of the atria.
The above analysis has been described using a single plane, namely the frontal plane characterized by the superior, inferior, right and left boundaries of the human body. In order for a physician to analyze the overall movement of the heart muscle during depolarization and repolarization, the physician needs to analyze the vector forces along another plane, namely the horizontal plane which is characterized by the posterior, anterior, right and left boundaries of the human body.
Much in the same manner as described above, six leads are positioned about the body to measure the electrical currents across the heart muscle in the horizontal plane. These leads are typically called the precordial leads and are represented as V1-V6, respectively. Using the same methodology as described above with respect to the frontal plane, the location and direction of the mean QRS vector in the horizontal plane can also be determined.
When the two planes are analyzed simultaneously, the mean QRS vector (and the other vectors) projects perpendicularly from the transition “plane” rather than the transition “line” of the single plane system. In other words, when the frontal plane and the horizontal plane are isolated and individually analyzed, the mean QRS transition appears as a line across the diameter of the circle. In actuality this “line” is actually a “plane” when both systems (frontal and horizontal) are analyzed simultaneously and the mean vectors (QRS, T and P) project perpendicularly from this plane into both systems.
As can be appreciated from the above summary, the analytical process of determining the resultant QRS vector and the other vectors can be quite cumbersome and requires a physician to interpret various graphs and/or solve various formulas which tend only to frustrate the diagnostic process and which can lead to erroneous conclusions if analyzed improperly. For simplicity, most physicians analyze each system individually at first and then combine the results. However, as often is the case, the determination of the mean vectors (QRS, T and P) in one plane is still both time consuming and somewhat confusing. Further, trying to determine how the mean vectors project into two planes and how the angles between the vectors relate can be even more confusing.
Moreover, even if a physician can adequately analyze the various graphs and solve the various formulas to arrive at a diagnosis, three-dimensional representation of the location of the mean QRS vectors (and the other vectors) must be mentally visualized which requires a high degree of mental agility and can lead to misdiagnosis. Further, mentally visualizing the angles between mean vectors would be virtually impossible for even the most skilled physician. The additional problem of how these vectors change in time over the QRS interval is believed to be nearly impossible to consider by the prior methods.
In the past, several attempts have been made at resolving the above problems. For example, 2-D vector cardiograms isolated the various signals from the leads and used several oscilloscopes to show the results in three planes (frontal, transverse (horizontal) and sagittal). This has been studied in great detail and many texts have been written to relate these diagrams to various heart maladies. However, as far as is known no one has ever attempted to display the signal as a series of 3-D vectors plotted at intermittent time intervals over the duration of the signal, much less represent these vectors on a single display and on a single 3-D coordinate system thereby producing a more easily identifiable 3-D view of the 12-Lead ECG signal or QRS complex as it progresses through the cardiac muscle over time.
As can be appreciated, the above issues are exacerbated during continual heart monitoring, e.g., monitoring patients in the telemetry unit of a hospital. For example and as mentioned above, continual heart monitoring utilizing a standard 12-lead display system is a demanding process. As a result, heart monitoring is usually automated such that slight changes in the electrical signal from the heart are typically registered. A set of conditions are programmed into the heart monitor and upon reaching a predetermined threshold a bell or buzzer alerts the hospital staff. As is often the case, false signals are generated which, as can be appreciated, can be a tremendous waste of hospital resources. For example, simple positional changes (i.e., a patient turns over to lie on his/her side) often trips the heart monitoring alarm to alert the staff of an ischemic condition. As can be appreciated, this can be stressful on the hospital staff.
It would therefore be desirable to provide a device which can overcome many of the aforesaid difficulties with diagnosing, analyzing and monitoring heart malfunctions and provide devices and methods which display heart maladies in an easily recognizable, distinguishable, consistent and effective manner allowing even an untrained observer to easily visualize, isolate and analyze common heart conditions.